You have to blunder check every move. You say you use less than 10 minutes in your 30 minute games. Sit on your hands while you play and each time you are about to move your piece, commit to spending a full 20 seconds blunder checking. Do this even for your first moves! The point is that you need to get into the habit of blunder checking. If you do this for enough games, you’ll start to notice that you’re blunder checking without even thinking about it.

Play slower. Not everyone is a whiz who can calculate 5 moves deep in 10 seconds. Just give it a try. Fire up an online game at 1 day or more PER MOVE and then look at the darn board off and on many times. Use the analysis board provided to try the move visually then make the next move for your opponent (flip the board, this is IMPORTANT to most humans) to see if you made a mistake.

I am a very slow thinker. I look at a game in the forums to comment on and try to go fast and tell people they should hang their queen or something. I play a game in 15 per side and my rating is maybe 1400. Put me in blitz and its closer to 1000. But given 3 days to move, or really 1+ is sufficient, and I do pretty well against 1800 level players. The difference in my ability when I have (and TAKE) time to look at the board is astounding.